I am currently running 8.1.2 on an iphone 6 plus. I have recently noticed that the internet became practically unusuable so i decided to reset my network settings. I tried signing back in to the same wifi I used on my other devices and it gave me the "incorrect password" error. So at first i thought of trying to forget and reconnect to the network on my iphone 5s(also running 8.1.2) to make sure i wasn't typing the weong password, but it worked. Next i tried to go into the router settings and copy the password onto my 6 plus but still could not connect. I've never gotten this problem before.
EDIT:
I will try and update to ios 9, if it still does not work. I'm going to apple to exchange and get back on how it goes
Update:
I decided to hard reset and restore my phone and the wifi started working again. The problem has been solved, i do not know how but i would appreciate anyone's answer might there be any future problems
Best Answer
I had that issue on an iphone 5 a bunch of times. All the other apple devices worked just fine. Very frustrating. There's a thread on one of the support forums with hundreds of replies from folks with similar issues but no real solutions. We replaced the router as well but that did not fix it. After the phone was replaced with same model it went away so I think it may have been a hardware issue. When I had the issue, a workaround I found was to use 5GHz only as the 2.4Ghz gave the issues but the 5GHz did not. HTH